1. Field of the Invention
The present invention relates to an ether compound and a cured resin which is prepared by using the ether compound, exhibits a small chromatic aberration, an excellent tint performance and an excellent antioxidation property and is useful in various applications, particularly as optical materials such as plastic lenses, prisms, optical fibers, substrates of information recording materials and filters, more particularly as a material of plastic lenses of glasses.
2. Description of the Related Arts
Plastic materials have widely been used as various optical materials, particularly as lenses of glasses, because of light weight and toughness. The properties required for optical materials, particularly for lenses of glasses, are a low specific gravity, properties which are occasionally referred to as optical properties i.e., a large refractive index and a large Abbe number, high heat resistance and large strength. A large refractive index is important to decrease the thickness of a lens. A large Abbe number is important to decrease chromatic aberration of a lens. High heat resistance and large strength are important to facilitate fabrication and also from the standpoint of safety. The antioxidation property is also important because optical materials such as plastic lenses must be treated under heating during annealing and coating and are colored by oxidation in these treatments. Tint performance is also required for facilitating production of sunglasses. A typical plastic optical material in early periods of conventional technology is diethylene glycol bisallyl carbonate. This material has a refractive index of about 1.5. Therefore, the thickness of a lens increases and, as a result, the weight cannot be reduced. Materials having a higher refractive index have been desired and various attempts have been made to raise the refractive index to a value of 1.6 or higher. Thermosetting optical materials having a thiourethane structure which are obtained by the reaction of a polythiol compound and a polyisocyanate compound have been proposed (Japanese Patent Publication Heisei 4(1992)-58489 and Japanese Patent Application Laid-Open No. Heisei 5(1993)-148340). Improvement in the refractive index and the Abbe number by the use of these optical materials having the thiourethane structure is insufficient although the improvement can be achieved to some degree. Technology to obtain a lens by polymerization of an epoxy resin or an episulfide resin in combination with a multi-functional compound has been proposed in the specifications of Japanese Patent Application Laid-Open No. Heisei 1(1989)-89615, Japanese Patent Application Laid-Open No. Heisei 3(1991)-81320 and International Patent Application Laid-Open No. wo8910575. However, the optical materials of the conventional technology which are obtained by curing an epoxy compound or an episulfide compound by polymerization have insufficient refractive indices, poor Abbe numbers and insufficient balances between the refractive index and the Abbe number. The thickness and the weight can be decreased by the conventional compounds containing sulfur to some degree. However, it is natural that an optical material having a still larger refractive index is desired. A material simultaneously exhibiting a large refractive index and a large Abbe number is also desired. Generally, the larger the refractive index, the smaller the Abbe number. Plastic materials obtained from conventional compounds such as diethylene glycol bisallyl carbonate, combinations of a polythiol compound and a polyisocyanate compound, epoxy compounds and episulfide compounds have the maximum Abbe number of about 50 to 55 when the refractive index is 1.50 to 1.55, about 40 when the refractive index is 1.60 and about 30 when the refractive index is 1.65. It is impossible to achieve an excellent balance between the refractive index and the Abbe number such as an Abbe number of about 40 or larger with a refractive index of 1.60 or an Abbe number of about 35 or larger with a refractive index of 1.65. The present inventors have disclosed optical materials comprising sulfur and carbon atoms as the main components and exhibiting an excellent balance between the refractive index and the Abbe number (for example, Japanese Patent Application Laid-Open No. Heisei 9(1997)-110979). However, some of these materials exhibit insufficient tint performance and antioxidation property.
An object of the present invention is to develop an optical material which has a decreased thickness and simultaneously exhibits a small chromatic aberration, an excellent tint performance and an excellent antioxidation property. Conventional optical materials which are, typically, materials obtained from a polythiol compound and an isocyanate compound or an episulfide compound have drawbacks in that improvement in the refractive index is limited, that an increase in the refractive index causes a decrease in the Abbe number, that a simultaneous improvement in the refractive index and the Abbe number by a structure containing sulfur in a large amount causes deterioration in the tint performance and the antioxidation property and that, as the result, it is difficult to simultaneously achieve an excellent balance between a sufficiently large refractive index and a sufficiently large Abbe number, an excellent tint performance and an excellent antioxidation property.
It was found that the above drawbacks can be overcome by:
(1) An ether compound having a structure represented by following formula (1) and at least one of a structure represented by following formula (2) and a structure represented by following formula (3): 
wherein X represents S or O, the average number of S represented by X is 50% or more of a total number of S and O constituting the three-membered rings, R4, R8 and R12 each represents a hydrocarbon group having 1 to 10 carbon atoms and R1 to R3, R5 to R7 and R9 to R11 each represents hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms;
(2) A bis(xcex2-epithioalkyl) ether compound represented by following formula (4): 
wherein R16 and R17 each represents a hydrocarbon group having 1 to 10 carbon atoms, R13 to R15 and R18 to R20 each represents hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms and X represents S or 0;
(3) A cured resin obtained by polymerizing a compound described in any of (1) and (2);
(4) A process for producing a cured resin comprising polymerizing a compound described in any of (1) and (2); and
(5) A lens comprising a cured resin described in (3).
In other words, it is found that the drawbacks of the conventional optical materials can be overcome when the cured resin of the present invention is used as the optical material.